The invention relates to a transmission system comprising at least a transmitter and a receiver, said receiver comprising a digital data demodulator. The invention also relates to a receiver and a demodulator designed for use in such a system.
The invention has important applications in the field of digital modulations.
The standardization project DVB-T (Digital Video Broadcasting-Terrestrial) as defined by the ETSI and relating to the distribution of digital TV programs by radio links describes an example of such a transmission system. In this project in particular multicarrier modulations which are used which renders it possible to use the characteristics of the transmission channel to the best advantage.
The use of multicarrier modulations leads to a particular problem. The multicarrier transmission technique consists in a frequency multiplexing of N carriers which are modulated by points of a constellation (for example, points of a QAM constellation). Each transmitted symbol (called FDM symbol, short for Frequency Division Multiplexing) thus corresponds to a block of N points, each point of the block modulating one of the N carriers. In the case of multicarrier transmissions, the sampling frequency of the transmitted signal is accordingly much higher than the frequency of the FDM symbols. At the demodulator level, certain demodulation functions are carried out at a frequency of the order of the sampling frequency of the received data, whereas other functions are carried out at a frequency of the order of the symbol frequency.
On the other hand, it is desirable to use a static communication model for managing the data exchanges inside such a demodulator. The use of a static communication model comprises the realization of given functions at given moments by executing programs which are repeated at one and the same frequency, so that data can be provided in a regular rhythm. This type of communication model is advantageous because it enables to guarantee that all the data are correctly transmitted and that accordingly the functions are correctly executed.
If the architecture of a demodulator for multicarrier modulations is a static architecture, the choice of the common repetition frequency must necessarily be the lowest frequency from among the frequencies which can be used, i.e. a frequency which is of the order of the symbol frequency. This means that a very large number of instructions must be stored in a memory. In particular, the instructions relating to the treatment of the various carriers will have to be stored as many times as there are carriers. This solution is extremely costly from a memory point of view.
The invention has for its object to provide a demodulator which provides a solution to this problem.
This object is achieved with a transmission system, a receiver, and a demodulator as described in the opening paragraphs which are characterized in that said demodulator comprises:
a first module designed for carrying out first demodulation functions in accordance with at least one first program which is repeated with a first frequency,
a second module capable of carrying out second demodulation functions in accordance with at least one second program which is repeated with a second frequency, and
an interface module for exchanging data between said modules.
The invention thus provides a separation of the demodulator architecture into two parts which are in communication through an interface. In the case of multicarrier modulations, one of these parts (the first module) is designed for carrying out functions which must be performed at a frequency of the order of the sampling frequency. The other part (the second module) is designed for carrying out functions which must be performed at a frequency of the order of the symbol frequency. The operating programs of the first module have a repetition frequency of the order of the sampling frequency. Consequently, the instructions relating to the processing of the various carriers are only stored in a memory a minimum number of times. The necessary memory length for the storage of said programs is accordingly reduced to a minimum.
The invention furthermore provides the advantage that an architecture is created which can be used for transmission systems which use monocarrier modulations, for example for transmission systems by cable or by satellite. In this type of system, the sampling frequency and the symbol frequency are of the same order of magnitude such that the problem described above does not arise. In that case, only one of the modules is used (the first module). The operating programs of this module have a repetition frequency of the order of the symbol frequency.